Oct 15 2013:
In nature, a lot of alpha predators and other animals that don't really have to worry about being predated upon sleep for hours on end (during the night as well as the day). It helps conserve energy, reduce stress, and is an ideal time for repair and maintenance work. Especially on the brain, which is quite maintenance intensive--if you ever go more than two-three days without sleep, you'll notice your metal performance deteriorating sharply.

Its not like you could do much at night other than standing watch in an ancient society anyway. Lack of proper night vision is a pain. Proper night vision would have been better obviously, but evolution works by random chance and trial and error, so we don't have that.
Its much easier to set up sentries for safety, and sleep the night away. Pre-industrial human sleep was once even divided into two intervals, where you'd be awake for an hour or two in the middle--ideal for sentries.

Oct 15 2013:
daytime and nighttime are very different, and requires a different set of senses. what works daytime, is suboptimal nighttime, and vice versa. it is costly to have senses that work both daytime and nighttime. therefore complex animals tend to specialize either for one or the other. and since there is an inactive period anyways, it is practical to move all sort of maintenance tasks to that period if at all possible.

fast forward a million years, and we live in cities, virtually separated from the wilderness. we could be active for 24 hours a day, we don't care about darkness. but million years of evolution left its mark, and won't go away that easily. it is a relic. i'm willing to pay a huge sum if show me how to fix it. not that huge, i'm not bill gates.